1. Technical Field
The present invention relates to a control system of an electric power steering apparatus for vehicles in which an electric motor generates a torque for assisting a steering torque generated through manipulation of a steering wheel by a driver and to a method for controlling the system.
2. Background Art
FIG. 9 is a block diagram showing a construction of a conventional electric power steering control system similar to, for example, a system described in the Japanese Patent Publication (unexamined) No. 186994/1995.
In the drawing, reference numeral 1 is a steering torque detector for detecting a steering torque generated in a steering wheel when a driver manipulates the steering wheel. Numeral 2 is a steering torque controller for computing an assisting (auxiliary) torque and outputting an assisting torque signal on the basis of an output of the steering torque detector 1. Numeral 3 is a motor speed detector for detecting a motor speed. Numeral 4 is a damping compensator for computing a damping compensation signal on the basis of the motor speed detected by the motor speed detector 3. Numeral 5 is a motor acceleration detector for detecting a motor acceleration using an output of the motor speed detector 3. Numeral 6 is an inertia compensator for computing an inertia compensation signal on the basis of the motor acceleration detected by the motor acceleration detector 5.
Numeral 7 is a judgment (determination) unit for judging whether or not the direction of the output of the steering torque detector 1 and that of the output of the motor speed detector 3 are the same. The result of judgment is outputted to each of the steering torque controller 2, the damping compensator 4, and the inertia compensator 6.
Numeral 8 is a first adder for computing a sum (i.e., this sum is a target torque) of the assisting torque signal, the damping compensation signal, and the inertia compensation signal. Numeral 9 is a motor current determiner for computing a target current signal from the target torque computed by the first adder 8. Numeral 10 is a motor in which a motor current value corresponding to an applied voltage is generated. By this motor 10, an assisting torque that is approximately in proportion to the motor current value is generated, and a steering mechanism not shown is driven. Numeral 11 is a motor current detector for detecting a current value of the motor 10. Numeral 12 is a second adder for obtaining a difference between the target current signal outputted by the motor current determiner 9 and the motor current value detected by the motor current detector 11. Numeral 13 is a motor drive for determining a voltage to be applied to the motor 10 on the basis of the difference between the target current signal computed by the motor current determiner 9 and the motor current value detected by the motor current detector 11. The motor drive 13 applies the determined voltage to the motor 10. Numeral 14 is a speed detector for detecting a speed of the vehicle and outputting the detected speed signal to the steering torque controller 2, the damping compensator 4, and the inertia compensator 6, respectively.
Described below is operation of the conventional electric power steering control system of above construction.
When a driver of a vehicle manipulates a steering wheel not shown, the steering torque detector 1 and outputted to the steering torque controller 2 measures a steering torque applied to the steering wheel. The steering torque controller 2 computes the assisting torque signal that is approximately in proportion to the output signal of the steering torque detector 1. Accordingly, the motor 10 is driven and an assisting torque is generated on the basis of the assisting torque signal, and a steering torque of the driver is assisted to reduce the steering torque that the driver feels.
At this time, the judgment unit 7 judges whether or not the direction of the output of the steering torque detector 1 and the direction of the output of the motor speed detector 3 are the same. If they are judged the same, the damping compensator 4 and the inertia compensator 6 do not work, but only the steering torque controller 2 works. The steering torque controller 2 determines an assisting torque signal according to the output of the steering torque detector 1 and the speed signal from the speed detector 14. A target torque is determined on the basis of the determined assisting torque signal, and the motor current determiner 9 determines a motor drive current.
In the case that the direction of the output of the steering torque detector 1 and the direction of the output of the motor speed detector 3 are not the same, the steering torque controller 2 does not work, but the damping compensator 4 and the inertia compensator 6 work. In this case, the target torque is determined on the basis of the outputs of the damping compensator 4 and the inertia compensator 6, and the motor current determiner 9 determines the motor drive current.
The direction of the target torque is arranged to be the same as that of rotation of the motor when the vehicle is driven at a low speed. The direction is arranged to be opposite to that of rotation of the motor when the vehicle is driven at a high speed. Therefore, when the driver is turning (steering) the steering wheel, the steering torque of the driver is assisted so as to reduce the torque necessary for the steering manipulation. When the driver is returning the steering wheel, the motor 10 is controlled so as to assist the steering wheel to return to the starting point if the vehicle is driven at a low speed and prevents the steering wheel from returning at an excessive speed of rotation if the vehicle is driven at a high speed.
The foregoing flow is shown in FIG. 10. The flow shown in this drawing is hereinafter referred to as main routine. For better understanding, FIG. 11 shows the relation between the directions of the steering torque caused by increase and decrease in speed of the vehicle and of the motor speed.
In general, the driver turns the steering wheel when the vehicle goes round a curve or an intersection. Then the driver returns the steering wheel utilizing a spontaneous returning force of the steering wheel due to road surface reaction torque of tires when the vehicle goes back to a straight running after going round the curve or the intersection. However, when the vehicle is driven at a low speed or when the vehicle is driven at a high speed but the steering wheel is steered a little, the road surface reaction torque of the tires is weak. Consequently, the road surface reaction torque is smaller than friction torque in the steering mechanism, and in many cases the steering wheel does not return when the vehicle goes back to the straight running. To cope with this problem, in the prior art shown in FIGS. 9 to 11, whether or not the output of the steering torque detector 1 and that of the motor speed detector 3 are the same is judged when the vehicle is driven at a low speed. If the outputs are not the same, the motor drive current is established so as to rotate the motor 10 in the same direction as the motor rotation, thereby the returning property of the steering wheel in driving at a low speed is improved. However, in most cases, it is necessary for the driver to add a torque to the steering wheel in order to return the steering wheel, which results in a phenomenon that steering sense or feeling is deteriorated.
In the prior art, when the steering wheel is manipulated within a range where the road surface reaction torque of the tires is small such as going round a curve at a low speed or going round a gentle curve at a high speed, the steering wheel stops and the motor 10 will not rotate unless the driver applies a certain torque in the direction of returning the steering wheel. That is, in this case, it is not possible for the judgment unit 7 to judge whether or not the direction of the output of the steering torque detector 1 and that of the output of the motor speed detector 3 are the same. Therefore it is not possible for the steering torque controller to establish the torque (motor driving current) so as to rotate the motor in the same direction as that of the motor rotation. Consequently, a problem exists in that returning property of the steering wheel is not improved because any electric current does not flow in the motor.
The present invention was made to resolve the above-discussed problems and has an object of obtaining an electric power steering apparatus capable of returning a steering wheel without application of a torque in returning direction of the steering wheel when the steering wheel is manipulated within a range wherein road surface reaction torque of tires is small such as a case of going round a curve at a low speed or a case of going round a gentle curve at a high speed, controlling returning property of the steering wheel by using a target steering angle (the target steering angle is 0xc2x0 when the driver wants to return the steering wheel to the original point) corresponding to the will of the driver, thereby improving the returning property of the steering wheel in any driving condition and improving convergence and damping performance after unhanding the steering wheel by using the steering angle. The invention also provides a method for controlling the electric power steering apparatus.
An electric power steering control system according to the invention comprises:
a motor for applying an assisting torque to a steering wheel used for manipulating running wheels of a vehicle;
steering torque detecting means for detecting a torque applied to the steering wheel by a driver as a steering torque;
motor angle detecting means for detecting a rotating angle of the motor;
road surface reaction torque estimating means for estimating a road surface reaction torque of the running wheels using the steering torque;
neutral point learning means for learning a neutral point of the steering wheel under the actual driving condition from the estimated road surface reaction torque and the rotating angle of the motor; and
steering angle compensation means for computing a return torque to return the steering wheel to the neutral point from a difference between the rotating angle of the motor and the learned neutral point and for converting the return torque into an electric current of the motor, whereby the return torque is applied to the steering wheel whenever the steering wheel is turned from the neutral point.
Another electric power steering control system according to the invention comprises:
a motor for applying an assisting torque to a steering wheel used for manipulating running wheels of a vehicle;
steering torque detecting means for detecting a torque applied to the steering wheel by a driver as a steering torque;
motor angle detecting means for detecting a rotating angle of the motor;
motor current setting means for setting a target value of an electric current flowing in the motor by receiving a torque command signal;
motor current detecting means for detecting a value of the electric current flowing in the motor;
road surface reaction torque estimating means for estimating a road surface reaction torque of the running wheels using the steering torque and the current of the motor;
neutral point learning means for learning a neutral point of the steering wheel under the current driving condition from the estimated road surface reaction torque and the rotating angle of the motor; and
steering angle compensation means for computing a return torque to return the steering wheel to the neutral point from a difference between the rotating angle of the motor and the learned neutral point and outputting the return torque to the motor current setting means, whereby the return torque is applied to the steering wheel whenever the steering wheel is turned from the neutral point.
It is preferable that the electric power steering control system is provided with:
speed detecting means for detecting speed of the vehicle; and
means for stopping operation of the neutral point learning means when the vehicle speed detected by the speed detecting means is smaller than a predetermined value.
It is also preferable that the electric power steering control system is provided with rotating direction detecting means for outputting different codes according to the rotating direction of the motor, and the neutral point learning means learns a neutral point in left steering and a neutral point in right steering respectively according to the codes outputted by the rotating direction detecting means at the time of learning the neutral point and subsequently learns a neutral point by computing an average of the neutral point in left steering and the neutral point in right steering.
It is also preferable that the electric power steering control system is provided with rotating direction detecting means for outputting different codes according to the rotating direction of the motor, and the neutral point learning means learns a neutral point on the basis of a value obtained by subtracting or adding a preliminarily stored friction torque of a steering mechanism according to the rotating direction from or to the estimated road surface reaction torque.
It is preferable that the electric power steering control system is provided with rotating speed detecting means for detecting a rotating speed of the motor or angular acceleration detecting means for detecting a rotating angular acceleration of the motor, and the neutral point learning means learns a neutral point when at least one of the rotating speed and the rotating angular acceleration of the motor is smaller than a predetermined value and does not learn the neutral point when the rotating speed or the rotating angular acceleration of the motor is larger than the predetermined value.
It is preferable that the steering angle compensating means is provided with a limiter for limiting an output of the steering angle compensating means to be within a predetermined level.
It is preferable that the electric power steering control system is provided with:
speed detecting means for detecting a vehicle speed;
a damping compensator for compensating a damping on the basis of speed of the motor; and
an inertia compensator for compensating an inertia on the basis of acceleration of the motor;
wherein the damping compensator and the inertia compensator work when the vehicle speed exceeds a predetermined level, and the damping compensator and the inertia compensator stop working when the vehicle speed does not exceed the predetermined level.
A method for controlling an electric power steering control system according to the invention includes:
a step of detecting a motor angle in which a rotating angle of a motor for applying an assisting torque to a steering wheel used for manipulating running wheels of a vehicle is detected;
a step of estimating a road surface reaction torque in which a road surface reaction torque is estimated using a steering torque applied to the steering wheel by a driver and a current signal of the motor;
a step of learning a neutral point in which a neutral point of the steering wheel is learned from the road surface reaction torque of the running wheels and the rotating angle of the motor; and
a step of compensating a steering angle in which a torque for returning the steering wheel to the neutral point is computed from the neutral point and the rotating angle of the motor.
It is preferable that the method for controlling an electric power steering control system includes:
a step of detecting a vehicle speed; and
a step of stopping an operation of the step of learning a neutral point when the vehicle speed is lower than a predetermined value.
It is also preferable that the method for controlling an electric power steering control system includes:
a step of detecting a rotating direction in which different codes are outputted according to the rotating direction of the motor; and
a step of learning a neutral point by learning a neutral point in left steering and a neutral point in right steering respectively according to the codes of the rotating direction outputted at the time of learning the neutral point and subsequently computing an average of the neutral point in left steering and the neutral point in right steering.
Since the invention is composed as described above, following advantages are performed.
The rotating angle of the steering wheel is detected using the motor angle in which the neutral point has been corrected on the basis of the signal of estimated road surface reaction torque. As a result, the neutral point of the steering wheel can be grasped without using a steering wheel angle sensor. The steering wheel return torque is generated on the basis of the grasped neutral point of the steering wheel. As a result, in the case that the steering wheel does not return such as going round a gentle curve, the steering wheel spontaneously returns without the driver""s returning operation of the steering wheel, and it is possible to construct a power steering control system superior in drive feeling.
The road surface reaction torque is estimated on the basis of the actual electric current detected by the detector. As a result, as compared with the case of using a target electric current, the neutral point of the steering wheel angle is learned more accurately even when there is an offset between the target value and the actual electric current.
It is arranged that the neutral point is not learned when the vehicle is driven at a low speed by using the vehicle speed detecting means. As a result, it is possible to improve accuracy in detecting the neutral point and generate a favorable and appropriate assisting torque of the motor.
The electric power steering control system is provided with the motor angle speed detecting means. The neutral point of the steering angle is learned by learning the neutral point in left steering and that in right steering according to the code of the motor angle speed outputted at the time of learning the neutral point and by computing an average of the neutral point in left steering and that in right steering. As a result, both in left and right steering, friction components of the steering mechanism of approximately the same intensity but in the opposite direction are automatically cancelled. Accuracy in detecting the neutral point is further improved without preliminarily grasping the intensity of friction of the steering mechanism, and as a result it is possible to generate a favorable and appropriate assisting torque of the motor at all times.
The neutral point is learned by subtracting the friction torque in the left and right steering as an offset quantity from the road surface reaction torque estimated by the road surface reaction torque estimating means. As a result, it is possible to further improve accuracy in detecting the neutral point and generate a favorable and appropriate assisting torque of the motor at all times.
The electric power steering control system is constructed so that the neutral point may be learned within the predetermined range of the motor angle speed or within the predetermined range of the motor angle acceleration. Therefore, it is possible to learn the neutral point only under the steering conditions that the neutral point can be learned most accurately, therefore accuracy in detecting the neutral point is not deteriorated. As a result, it is possible to generate a favorable and appropriate assisting torque of the motor at all times.
The electric power steering control system has a construction in which the limiter limits the output for correcting and controlling the fundamental target current. As a result, even if the neutral point of the steering angle is erroneously learned, it is possible to generate an assisting torque with which the driver can easily recover the neutral point, thereby improving safety.
The electric power steering control system is provided with the damping compensator and the inertia compensator, and they are controlled to work or not to work according to the speed of the vehicle. As a result, it is possible to obtain an electric power steering control system superior in drive feeling.
The method for controlling an electric power steering control system according to the invention includes the steps of estimating a road surface reaction torque, learning a neutral point of the steering wheel from the road surface reaction torque, correcting a rotating angle of the motor on the basis of this neutral point, and compensating a steering angle by computing a torque for returning the steering wheel to the neutral point from the rotating angle. As a result, it is possible to generate a torque for returning the steering wheel to the neutral point without using a steering wheel angle sensor and without driver""s returning manipulation of the steering wheel.
The step of learning a neutral point is stopped when the speed of the vehicle is lower than the predetermined value. As a result, it is possible to prevent the neutral point from erroneous judging and control the electric power steering control system more accurately.
The neutral point is learned by learning the neutral point in left steering and that in right steering according to the rotating direction of the motor and by using an average of them as the original neutral point. As a result, it is possible to recognize the neutral point more accurately.